Turbine engines include a compressor assembly, a combustor assembly, and a turbine assembly. The compressor compresses ambient air, which is channeled into the combustor where it is mixed with fuel and burned, creating a heated working gas. The working gas can reach temperatures of about 2000-3000° F., and is expanded through the turbine assembly. The turbine assembly has a series of circular arrays of rotating blades attached to a central rotating shaft. A circular array of stationary vanes is mounted in the turbine casing just upstream of each array of rotating blades. The stationary vanes are airfoils that redirect the gas flow for optimum aerodynamic effect on the next array of rotating blades. Expansion of the working gas through the rows of rotating blades and stationary vanes causes a transfer of energy from the working gas to the rotating assembly, causing rotation of the shaft, which drives the compressor.
The vane assemblies may include an outer platform element or shroud segment connected to one end of the vane and attached to the turbine casing, and an inner platform element connected to an opposite end of the vane. The outer platform elements are positioned adjacent to each other to define an outer shroud ring, and the inner platform elements may be located adjacent to each other to define an inner shroud ring. The outer and inner shroud rings define an annular working gas flow channel between them.
Vane assemblies may have passageways for a cooling fluid such as air. The coolant may be routed from an outer plenum, through the vane, and into an inner plenum attached to the inner platform elements. The vanes are subject to mechanical loads from aerodynamic forces on them while acting as cantilever supports for the inner platform elements and inner plenum. Thus, problems arise in assembling vanes with both the required mechanical strength and thermal endurance.
Attempts have been made to form vane platforms and vane cores of metal with a ceramic matrix composite (hereinafter “CMC”) cover layer. However, ceramic materials present certain problems that have heretofore prohibited their widespread use for constructing part or all of the vane. Firstly, ceramic materials do not have the tensile strength of metallic materials. Secondly, due to its usually relatively low ductility, ceramic material has a tendency to crack under the impact of severe or suddenly applied thermal shock or stresses such as may occur in advanced lightweight aircraft gas turbines. Thirdly, where it is desired to strengthen the blade or vane structurally, such as by means of metallic body or strut member or members, the dissimilar characteristics of the metal body and the ceramic materials in the areas of ductility, thermal conductivity, and brittleness, for example, create additional problems concerning how to mate these materials in an integral airfoil construction.
Furthermore, forming CMC airfoils, such as by wet layering on a metal core, is unsatisfactory, because curing of CMC requires temperatures that damage metal. Also CMC has a different coefficient of thermal expansion than metal, resulting in separation of the airfoil from the metal during turbine operation. CMC or superalloy airfoils may be formed separately and then assembled over the metal core, but this involves problems with assembly. If an inner and outer platform and vane core are cast integrally, there is no way to slide CMC cover elements over them. Thus, attempts have been made to form CMC airfoils split into halves, connecting the halves over the vane core. However, this results in a ceramic seam, which must be cured in a separate high-temperature step that can damage metal and may cause lines of weakness in the airfoil. If the platforms and vane are cast separately it is challenging to mechanically connect them securely enough to withstand the cantilevered aerodynamic forces and vibrational accelerations. It is also challenging to mount a CMC airfoil over a metal vane core securely in a way that accommodates differential thermal expansion without allowing vibration.
Accordingly, there is a need for improved turbine engines with CMC vanes and methods for manufacturing the same. Furthermore, other desirable features and characteristics of the inventive subject matter will become apparent from the subsequent detailed description of the inventive subject matter and the appended claims, taken in conjunction with the accompanying drawings and this background of the inventive subject matter.